The output devices of a personal computer often include a graphics adapter and a monochrome display. The graphics adapter (GA) contains a bitmap memory that is accessible by the computer's central processing unit (CPU) and the GA's CPU. Each bit in the bitmap memory corresponds to one pixel on the display screen. To display data, the GA CPU reads the bitmap memory. If a bit is 1 then the GA turns the corresponding pixel on. If a bit is 0 then the GA turns the corresponding pixel off. By changing the contents of the bitmap memory, a computer program can effect a change on the display screen.
To accommodate color displays the GA needs to be more sophisticated. A single bit in bitmap memory per screen pixel is not sufficient to represent more than two colors. If four colors are to be displayed, then two bits per pixel are needed; if eight colors are to be displayed, then three bits per pixel are needed; if sixteen colors are to be displayed, then four bits per pixel are needed; and so on. Each bit per pixel is conceptually considered to be in a separate plane, with a one bit per pixel bitmap maintained for each plane. FIG. 1 illustrates a bitmap with four planes. The GA CPU will read the 4 bits for each pixel from each of the four planes and turn the appropriate color on for that pixel on the screen.
The GA bitmap memory is typically an 8-bit byte (a byte is a sequence of adjacent binary digits operated upon as a unit in a computer) that is, eight bits can be written to the bitmap memory at a time. To fill an entire bitmap memory in a conventional computer system, the computer CPU would generally for each plane write each byte. Thus, the total number of byte output to the GA is the number of planes times the number of bytes per scan line times the number of scan line.